Method of establishing the residual useful life of contacts in switchgear and associated arrangement

ABSTRACT

To determine the remaining lifetime of contactor contacts, the contact spring action at the clearance gap can be determined as a substitute criterion for contact erosion, and to determine the erosion of the contact points, the change in spring action during the shutdown cycle can be measured and converted to the remaining lifetime, for which purpose, the time of the armature movement from the start of the armature movement to the start of contact opening is measured with the solenoid actuator having an armature with solenoids and associated yoke. The measured values of the time signal t k  of contact opening on the load side of the switching device monitored and the time signal t A  are determined by voltageless signaling of the start of armature movement. In particular for use in three-phase systems, the switching voltage is measured as a voltage change at an artificial neutral point. In the respective arrangement, a voltageless signal line is provided between the switching device and analyzer unit.

FIELD OF THE INVENTION

The present invention relates to a method of determining the remaininglifetime of contacts in switchgear, in particular contractor contacts.The present invention also relates to the respective arrangement forcarrying out the method, with an analyzer unit for displaying theremaining lifetime.

BACKGROUND INFORMATION

In German Patent Application No. 44 27 066 (not prior art), theremaining lifetime of a contactor in the shutdown cycle is derived fromthe difference in time between the start of the armature openingmovement and the start of contact opening. Using an analysis algorithm,a microprocessor then determines from the time difference value thepresent value of the contact spring action, which decreases from itsvalue when new (=100% remaining lifetime) to its minimum value (=0%remaining lifetime) due to contact erosion. The time signals requiredfor this are detected first by interrupting an auxiliary circuit overthe armature and yoke of the solenoid actuator and also by the contactvoltage at the main contacts and are converted to well-defined voltagepulses, for which purpose measuring leads must be attached.

Attaching measuring leads (six leads for three-phase current) foranalysis of contact voltages may be problematical inasmuch as

a) the possibility of vagabond voltage forming from the infeed side tothe load side of the contactor cannot be ruled out,

b) the required insulation voltage endurance (8 kV) results in a highercost for the analysis circuit, and

c) integrating the measuring leads into the contactor and connectingthem to a plug-in connector necessitates design and safety-relatedchanges.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and therespective arrangement, wherein the start of contact opening need not bedetermined over measuring leads on both the feed and load ends of themain circuit.

This object is achieved according to the present invention by measuredvalue acquisition on the contact gap on the load side of the monitoredswitching device and by voltageless signaling of the start of armaturemovement. For use in three-phase systems, the start of contact openingof the contact points with the greatest erosion of one of the switchingpoles is preferably detected by measuring the switching voltage as thechange in voltage at an artificial neutral point on the load side of theswitching device monitored, from which it is then possible to determinethe remaining lifetime of the main contacts of the contactor in additionto the start of armature movement.

In the respective arrangement with an analyzer unit for displaying theremaining lifetime, there is a voltageless signal line on the armatureand yoke of the solenoid actuator of the switching device between theswitching device and the analyzer unit. The analyzer unit is thuslocated between the switching device and the electric consumer on theload side.

To reduce the technical complexity, it is thus no longer necessary tomonitor each main circuit individually with regard to contact erosion inthree-phase systems in particular, but instead only the spring action ofthe most eroded contacts of one of the three switching poles is measuredto determine the remaining lifetime of the main contacts of thecontactor. Furthermore, it is possible to determine the remaininglifetime without a strict spatial correlation with the contactor, thestart of the armature opening movement being signaled to the analyzerunit over a voltageless signal line as contact interruption betweenarmature and yoke.

Voltageless signaling in the aforementioned context is understood torefer to electric contacting between armature and yoke, in contrast witha voltage signal, such as the contact voltage on the main contacts.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a device for determining a remaining lifetime of contactorsin a shutdown cycle according to the present invention.

FIG. 2 shows a circuit for generating a time signal for a first maincircuit of contactors to clear in the shutdown cycle in a three-phasesystem.

FIG. 3 shows a device for determining the remaining lifetime of areversing contactor circuit.

FIG. 4 shows a device for determining the remaining lifetime ofcontactors in the shutdown cycle in d.c. systems.

FIG. 5 shows a three-phase system including a four-pole switching devicefor three phases of a three phase current and a neutral conductor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram of a device for determining theremaining lifetime and associating it with a contactor 1. Analyzer unit100 is located between contactor 1 and electric consumer 20, e.g., amotor, on load side 10, and it is contacted with external conductors L1,L2, L3 over a first monitoring module 101 for detecting contact opening.A two-wire communication line 8 connects armature/yoke contact 7 ofcontactor 1 to a second monitoring module 102 for detecting opening ofthe armature. A microprocessor 105 determines the instantaneous contactspring action from the time signals supplied by monitoring modules 101and 102 and determines from this the remaining electrical lifetime ofthe main contacts.

The value determined by analyzer unit 100 for the remaining lifetime isdisplayed on an output unit 106 and can be output over a bus system forfurther processing.

As shown by control measurements on a contactor with the armature/yokecontact brought out, the time signals from which the remaining lifetimeis determined are subject to time fluctuations due to mechanicaltolerances and decay of the magnetic force. The time difference betweenthe time signals may therefore differ by a few {fraction (1/10)} msbetween two successive analyses. To avoid a corresponding fluctuation inthe output quantity, the remaining lifetime is determined on the basisof a sliding average, e.g., the last ten measurements. Therefore, anaccuracy of {fraction (1/10)} mm is considered realistic in determiningthe contact spring action. Faulty analyses in determining the timedifference can be prevented by analyzing only those time signals withina predetermined time window.

FIG. 2 shows an example of a circuit for generating a time signal t_(k)at the start of contact opening of the most eroded main contacts. Theessential characteristic of this circuit is that it measures the contactvoltages (arc voltage) of a triple-pole switching device in athree-phase system at “artificial” neutral point 15. For the contactvoltages, i.e., the arc voltage at interconnection point 15 of theoutput lines over fine-wire fuses 11 and resistors 12 (R=160 kΩ), thefollowing equations hold:

U₁+U₂+U₃=O, I₁+I₂+I₃=O

U₁−U_(STP)=R * I₁+L * d/dt(I₁)+UB₁

 U₂−U_(STP)=R * I₂+L * d/dt(I₂)+UB₂

U₃−U_(STP)=R * I₃+L * d/dt (I₃)+UB₃

Total:U_(STP)=−(UB₁+UB₂+UB₃)/3, where the following symbology has beenselected:

U_(i)=phase voltages

I_(i)=phase currents,

U_(Bi)=arc voltages,

i=1, 2, 3

U_(STP)=neutral point displacement voltage,

R=ohmic load,

L=inductive load.

With the main contacts of the contactor closed (UB₁=UB₂=UB₃=0), theneutral point displacement voltage would have to be 0 volt. In fact,however, the real phase voltages do not correspond to ideal sinusoidalvoltages, so that the total of the phase voltages differs from zero andthe neutral-to-ground voltage fluctuates around the voltage zero line.This signal noise can be reduced by a high-pass filter 16 (e.g., whereC=3 nF, R_(parallel)=500 kΩ) so that a signal-to-noise ratio of >10 isachieved. Electronic frame potential M can be picked off over ameasuring shunt (e.g., R_(Meβ)=10 kΩ). Time signal t_(k), i.e. thevoltage signal at “artificial” neutral point 15, is processed accordingto its polarity by one of two comparators 18 and 18′ whose outputs arecoupled to the signal output of the monitoring module for contactopening via an OR gate 19.

FIG. 3 shows a device for determining the remaining lifetime on theexample of a contactor reversing starter with two contactors 1 and 2;for application in three-phase motors, there are many different terminalconditions for controlling the speeds and direction of rotation.

The main circuits switched by contactors 1 and 2 correspond in basicdesign to the main circuits according to FIG. 1 or 3. There is usuallyan overload relay 210 on the load side of contactor 1 or 2 to protectthe motor load. It is therefore expedient to integrate overload relays210 and the device for determining remaining lifetime into a commoncontrol unit. As described in German Patent Application No. 44 27 006,this control unit could have additional functions for monitoring thecircuit state, so that this would ultimately result in a “general”control unit 200 for monitoring the entire electric system.

In the example in FIG. 3, only one second measuring channel is needed onmonitoring module 202 for armature opening to detect the remaininglifetime of both contactors 1 and 2. Microprocessor 205 attributes thecalculated lifetime to the contactor represented by the signalingmeasuring channel.

In three-phase systems, not only three-pole consumers, but alsofour-pole consumers, e.g., ohmic loads, are connected to the system anddisconnected from it by electric switchgear. The electric switchgearhave four switching poles, three of which are connected to externalconductors L1, L2, L3, while the fourth switching pole is connected tothe neutral conductor. An example of such a system is shown in FIG. 5,which illustrates a three-phase system including a four-pole switchingdevice for three phases of a three phase current and a neutralconductor. The system of FIG. 5 is similar to that of FIG. 2, so adescription of those elements that are common to both Figures shall notbe undertaken here. As with FIG. 2, which connects a fine-wire fuse 11and resistor 12 between each respective one of conductors L1-L3 andneutral point 15, FIG. 5 also includes a fine-wire fuse 11 and resistor12 connected between conductor N and the line on which neutral point 15is located. Each of these four switching poles is subject to contacterosion when the four-pole consumer is connected and disconnected, sothat wear on all contact points must be monitored, and the remaininglifetime must be determined according to the most eroded contact points.

The latter is achieved by measuring the switching voltage of one of thethree switching poles connected to external conductors L1, L2, L3 atartificial neutral point 15 and measuring the switching voltage of thefourth switching pole connected to neutral conductor N on neutralconductor N. Both the voltage of artificial neutral point 15 and thevoltage of neutral conductor N are detected on load side 10 of monitoredswitching device 1, and the error voltage, the difference between thetwo voltage values, is analyzed as the switching voltage of the first,most eroded contact tips to clear.

FIG. 4 shows a device for detecting the remaining lifetime of contactorsin d.c. systems. Depending on the system d.c. voltage and whether or notthe d.c. system is grounded, the contact gaps are usually connected inseries, and the connection of the electric system is designed with asingle pole or two poles. To obtain a uniform terminal condition for thetest connectors for monitoring contact opening and to rule out vagabondvoltage from the infeed side to the load side of contactor 1, themeasuring leads are connected to load side 10.

In the exemplary embodiment in FIG. 4, a monitoring module 300 hasindividual circuit elements 31 through 38. Specifically, it shows acontactor 1 with load side 10, a d.c. motor 20 and output lines 30 forconnecting monitoring module 300. It contains two fine-wire fuses 31, anRC combination 32 (C=0.22 MF, R₁=1 kΩ), a Zener diode 33, a resistor 34(R₂=330 Ω) and an optical coupler 35, whose output is connected tovoltage U across a resistor 36 (R=106 kΩ).

Blocking capacitor C in monitoring module 300 for contact opening servesto suppress the d.c. component; associated limiting resistors R1 and R2with the Zener diode serve to limit the voltage, and optical coupler 35in particular is for voltageless measurement of the contact voltage. Amicroprocessor 305 determines the contact spring action from time signalt_(k) of contact opening in the delay to the time signal of armatureopening, and from this it determines the remaining lifetime of the maincontacts of the contactor.

In all exemplary embodiments, the values determined by themicroprocessors can be displayed directly on associated output units orsent to a system for data transmission, in particular a bus system, forfurther analysis.

What is claimed is:
 1. A method for determining a remaining lifetimevalue of contacts in a switchgear, comprising the steps of: determininga contact spring action at a contact gap; during a shutdown cycle,measuring each change in the contact spring action to determine acontact erosion, including the steps of measuring a run-time value of anarmature path from a first start of an armature movement in a contactorsolenoid actuator to a second start of an opening of at least one of thecontacts, the measuring step being performed on a load side of theswitchgear, the first start of the armature being determined by avoltageless signaling of a start of the armature movement, determining apath length as a function of the run-time value, and determining thechange in contact spring action by a change in the path length;determining an erosion of at least one of the contacts as a function ofthe change in the contact spring action; and determining the remaininglifetime value of the at least one of the contacts as a function of theerosion.
 2. The method according to claim 1, wherein the contacts arecontactor contacts.
 3. The method according to claim 1, wherein thecontacts are driven by the armature, a solenoid and a yoke.
 4. Themethod according to claim 1, wherein the method is used in a three-phasesystem, the method further comprising the steps of: determining aswitching voltage as a function of a voltage change at an artificialneutral point on the load side of the switchgear; detecting the secondstart of the opening of the at least one of the contacts as a functionof the determining the switching voltage step, the at least one of thecontacts having a first erosion, the first erosion being a greatesterosion as compared to respective erosions of others of the contacts;and determining the remaining lifetime value as a function of thedetecting step.
 5. The method according to claim 4, wherein the at leastone of the contacts are mains contacts of a contactor.
 6. The method ofclaim 1, wherein the second start of an opening of at least one of thecontacts is determined by measuring a voltage rise of at least one ofthe contacts.
 7. The method of claim 6, wherein the measuring of thevoltage rise is performed on a load side of the switchgear.
 8. Themethod of claim 1, wherein the path length is a length of the armaturepath.
 9. An arrangement for determining a remaining lifetime value ofcontacts in a switchgear, comprising: an analyzer unit determining anddisplaying the remaining lifetime value; and a voltageless signal linearranged between the switchgear and the analyzer unit, the voltagelesssignal line provided to an armature and a yoke, a solenoid of theswitchgear including the yoke.
 10. The arrangement according to claim 9,wherein the contacts are contactor contacts.
 11. The arrangementaccording to claim 9, wherein the analyzer unit is positioned on a loadside between the switchgear and an electric consumer.
 12. Thearrangement according to claim 9, wherein the analyzer unit includes afirst monitoring module determining a start of a movement of an armatureto generate a first time signal, and a second monitoring moduledetermining a start of an opening of at least one of the contacts togenerate a second time signal.
 13. The arrangement according to claim12, wherein the analyzer unit includes a microprocessor determining acontact spring action as a function of the first time signal and thesecond time signal.
 14. The arrangement according to claim 9, whereinthe arrangement is a part of a three-phase system including a three-poleswitching device, the analyzer unit further including: a circuitarrangement generating a time signal at a start of an opening of atleast one of the contacts, the at least one of the contacts having agreatest erosion as compared to respective erosions of others of thecontacts, the circuit arrangement further measuring a contact voltage atan artificial neutral point.
 15. The arrangement according to claim 9,wherein the arrangement is included in a three-phase system, thethree-phase system including a four-pole switching device, the four-poleswitching device including three external conductors and a neutralconductor, the first arrangement further including: a circuitarrangement generating a time signal at a start of an opening of atleast one of the contacts, the at least one of the contacts having agreatest erosion as compared to respective erosions of others of thecontacts, the circuit arrangement further detecting a contact voltage ofthe four-pole switching device by measuring a voltage between anartificial neutral point and a voltage of the neutral conductor, theneutral conductor being on a load side of the switching device as areference potential of a resistor at a frame potential.
 16. Thearrangement according to claim 9, wherein the arrangement is included ina three-phase motor arrangement, the three-phase motor arrangementincluding an overload relay to protect a load of a motor, the overloadrelay and the analyzer unit being integrated into a common controldevice for detecting the remaining lifetime value.
 17. The arrangementaccording to claim 9, further comprising: contact gaps connected inseries for one of a single-pole and two-pole connection to a d.c.system; and measuring leads connected to a load of the d.c. system. 18.The arrangement according to claim 9, the analyzer unit including: amonitoring module including a blocking capacitor for suppressing a d.c.component, limiting resistors, a zener diode for voltage limiting, andan optical coupler for voltageless measurement of a contact voltage. 19.The arrangement according to claim 9, further comprising: a system fordata transmission coupled to the analyzer unit.
 20. The arrangementaccording to claim 19, wherein the system for data transmission is a bussystem.
 21. The arrangement of claim 9, wherein the analyzer further:determines a contact spring action at a contact gap; during a shutdowncycle, measures each change in the contact spring action to determine acontact erosion, including measuring a run time value of an armaturepath from a first start of an armature movement in a contactor solenoidactuator to a second start of an opening of at least one of thecontacts, the measuring being performed on a load side of theswitchgear, the first start of the armature being determined by avoltageless signaling of a start of the armature movement, determining apath length as a function of the run-time value, and determining thechange in contact spring action by a change in the path length;determines an erosion of at least one of the contacts as a function ofthe change in the contact spring action; and determines the remaininglifetime value of the at least one of the contacts as a function of theerosion.
 22. A method for determining a remaining lifetime value ofcontacts in a switchgear, comprising the steps of: measuring a change incontact spring action to determine a contact erosion, including thesteps of measuring a time difference from a first start of an armaturemovement in a contactor solenoid actuator to a second start of anopening of at least one of the contacts, the measuring step beingperformed on a load side of the switchgear, the first start of thearmature being determined by a voltageless signaling of a start of thearmature movement, determining an erosion of at least one of thecontacts as a function of the change in the contact spring action; anddetermining the remaining lifetime value of the at least one of thecontacts as a function of the erosion.
 23. The method of claim 22,wherein the second start of an opening of at least one of the contactsis determined by measuring a voltage rise of at least one of thecontacts.
 24. The method of claim 23, wherein the measuring of thevoltage rise is performed on a load side of the switchgear.
 25. Themethod according to claim 22, wherein the method is used in athree-phase system, the method further comprising the steps of:determining a switching voltage as a function of a voltage change at anartificial neutral point on the load side of the switchgear; detectingthe second start of the opening of the at least one of the contacts as afunction of the determining the switching voltage step, the at least oneof the contacts having a first erosion, the first erosion being agreatest erosion as compared to respective erosions of others of thecontacts; and determining the remaining lifetime value as a function ofthe detecting step.